In the case of helicopters which have to alight on a moving platform, it is already known from French Patent No. 1,241,147 to equip their landing gear with a controlled-adhesion device, comprising a large number of suction cups which are mounted to be retractable at the lower part of buoyancy tanks with which the helicopter is equipped. A large number of these suction cups are carried by a rubberized cloth closing the lower opening of a semicylindrical rigid shell, which is open towards the bottom and is rigidly mounted within the lower part of each buoyancy tank. In the recess delimited between the suction-cup-carrying rubberized cloth and the rigid shell, within the latter, there is disposed a sealed bladder made integral in places, on the one hand with the rigid shell and on the other hand with the cloth, which exhibits numerous openings, laterally in relation to the central suction cups, each one of which is likewise pierced by a small central channel. The sealed bladder may be pressurized from a compressed air connection on the outlet of the compressor of a turbine of the helicopter, or placed under reduced pressure by aspiration of the air which it contains by means of an ejector, the central nozzle of which is directly supplied from this air connection. The pressurization of the bladder ensures its inflation in the form of a cylindrical bulge, which pushes back the suction-cup-carrying cloth and the suction cups projecting towards the exterior of the rigid shell, i.e. projecting under the corresponding buoyancy tank and, simultaneously, the occultation of the small central channels for placing the suction cups in communication with atmosphere. Conversely, the placing of the bladder under reduced pressure retracts the suction-cup-carrying cloth and the suction cups towards the interior of the shell, and ensures the placing of the small central channels of suction cups in communication with the atmosphere.
Before executing the approach of the helicopter for alighting, the pilot ensures that the bladder is pressurized, and thus that the suction cups are moved out, with a view to the alighting, and are ready to adhere to the platform. At the moment of the impact on alighting, the adhesion of the suction cups on the platform takes place quasi-instantaneously as a result of the expulsion of the air via the edges of the latter in the course of their crushing under the effect of the distributed weight of the helicopter; this prevents any bounce and fixes the helicopter to the platform, the large number of suction cups permitting the alleviation of any possible lack of adhesion of some of them, on account of the quality of the surface condition of the platform or of its soiling.
In order to take off, the pilot instructs the depressurization of the bladder when he pulls on the control for the general pitch of the helicopter, in such a way that this depressurization ensures the simultaneous placing of all the suction cups in communication with atmosphere, at the same time as the retraction of the suction-cup-carrying cloth towards the interior of the rigid shell, in such a way that the helicopter is instantaneously released from the platform.
Thus, this device with controlled adhesion constitutes a device for fixing the helicopter on the platform, or alternatively an independent means for rapid docking, comprising numerous anchoring points, each one of which corresponds to a suction cup, and for which the docking and the release are simultaneous. It results from this that such a device effectively ensures an increase of stability of the helicopter set down, and thus an improvement in the stability against overturning of this helicopter, but only under static conditions. Such a device is unusable during hauling, and it even opposes the latter, when it is active, i.e., when the adhesion of the suction cups is utilized. Consequently, in order to permit hauling, it is necessary to disable this device, in such a way the improved stability which it procured under static conditions is then no longer ensured. It is therefore necessary to use another device, such as a trolley, for hauling, this handling trolley having to be slid under the helicopter which has been set down, while the bladder is maintained under pressure, the handling trolley being adapted to permit the movement of the helicopter once the bladder has been deflated, and consequently the suction cups retracted. Such an auxiliary trolley is absolutely essential since, in the case where the helicopter comprises a wheeled landing gear, the undercarriages of this gear cannot be used, on account of the fact that, in this case, the use of the controlled-adhesion device assumes specifically that the wheeled landing gear is retracted or cleared, with a view to utilization of the corresponding helicopter on a moving platform.
In fact, the controlled-adhesion device according to the French patent can be used only on helicopters equipped with a gear with floats, or with a wheeled gear subject to the condition that the helicopter is furthermore equipped with an emergency buoyancy device with buoyancy tanks disposed at its lower part, or alternatively that the helicopter is a hydro or amphibious helicopter, but in these last cases the wheeled landing gear must always be retracted or cleared in order to make it possible for the device with retractable suction cups to be used.
In conclusion, the device for controlled adhesion by retractable suction cups according to the French patent is functionally incompatible with a tricycle landing gear, of which the two principal undercarriages and the auxiliary front or rear undercarriage are effectively used for alighting and takeoff on a moving platform.
French Patent No. 2,544,280 discloses a landing gear, especially for an amphibious helicopter, the waterline of which is provided by the fuselage per se of the helicopter, and for which it is necessary to ensure the transverse balance. The landing gear is designed in such a manner as to ensure, on the one hand, the normal function of undercarriage on land or on a platform, and, on the other hand, the transverse stabilization of the helicopter on the water or under flood conditions, to the extent that the waterline is provided by the fuselage of the helicopter. To this end, each one of the two principal and lateral undercarriages is articulated on the fuselage about an axis inclined in the horizontal plane and the corresponding leg of the undercarriage is provided with a wheel and with a fairing in the form of a float, and each principal undercarriage is maneuvered by a control device permitting the movement thereof from a low position, suitable for alighting or takeoff on land or on a platform, and a high position, in which the float fairing ensures lateral stability under flood conditions and presents a correct incidence for hydroplaning.
It is clear that such a device increases the lateral stability of the helicopter solely when the latter is set down on the water since, when it is set down on land, or on a rigid platform, the wheeled undercarriages which constitute the lateral stabilizers under flood conditions are used in a conventional manner, and therefore do not procure any increase or improvement in stability.
On the other hand, numerous constructions are known of devices or of toolings intended exclusively to facilitate the ground movements of helicopters equipped solely with a landing gear with skids.
By way of example, British Patent No. 1,382,265 discloses a towing trolley comprising a frame mounted on wheels at each end, which trolley is equipped with a direction control at one end and moved by a motor which drives the wheels at the other end, the frame supporting a haulage device for producing a relative movement between the trolley and a helicopter with skids, and the trolley is equipped with two tracks, which are preferably equipped with rollers, to receive the skids of the helicopter, the haulage device being preferably driven by the same motor as the wheels of the trolley.
It is clear that such a trolley is a relatively heavy and bulky device, since not only must it be equipped with a motor and with its power supply, to move the helicopter, but it must also carry the latter, and for this purpose have available assistance in the lifting of the helicopter for loading thereof onto the trolley. Moreover, such a carrying trolley, which is specifically adapted for the movement of helicopters with skids, does not procure any increase in the lateral stability of the helicopter, in particular during loading onto the trolley, on account of the fact that the front or the rear of the helicopter must be slightly lifted, and even in a general way, when the helicopter rests on the carrying trolley, under static conditions just as during the hauling of the trolley, on account of the fact that the points of contact of the carrying trolley with the ground, namely the wheels of the carrying trolley, are disposed within the skids, and therefore closer to the longitudinal and median plane of the helicopter and, with regard to the points of contact at the front of the trolley, closer to the center of gravity of the helicopter. Moreover the center of gravity of the helicopter is elevated in relation to its position when the helicopter is resting on its skids. In conclusion, the stability, especially the transverse stability, of a helicopter with a landing gear with skids is diminished when the latter is resting on such a ground movement trolley.
The same applies when use is made of a trolley according to U.S. Pat. No. 4,033,422, which describes a small trolley, the self-powered frame of which is equipped with at least two coaxial wheels, with a motor for driving the wheels and with a jack lifting system intended to come into contact with a stable point on the lower face of the structure of the helicopter. This frame is equipped with a direction control arm, the operating handle of which is equipped with the components for controlling the drive motor and the lifting jack, and this small trolley is used in combination with two auxiliary wheeled gears, each one of which is intended to be fixed to the back of one of the two skids of the helicopter.
Such a handling trolley is relatively costly, heavy and complex, on account of the fact that it carries electrical batteries for the supply to the motor for driving the wheels, as well as to a motor for driving a pump of a hydraulic circuit for supplying power to the lifting jack. As previously, the lateral stability of the helicopter received by the handling trolley and its auxiliary rear rolling gears is diminished under static conditions, just as when hauling, on account of the "three point" support on the ground, at the location, on the one hand, of the wheels of the trolley, which are disposed at the front between the two skids and, on the other hand, at the location of the auxiliary rolling gears at the rear of the skids.
In conclusion, such handling trolleys are unusable on the moving platforms, on account of the fact that they diminish the stability, especially the lateral stability, of helicopters with skids which they transport and, on the other hand, they are specifically adapted for helicopters with a landing gear with skids, and cannot be used on helicopters having a tricycle landing gear, the undercarriages of which are used on alighting, just as on takeoff, on such moving platforms.
It should furthermore be noted that such handling trolleys, which slide under the helicopters with skids, are incompatible with the majority of the alighting aid devices, for example of the "grid-harpoon" type, with which the moving platforms may advantageously be provided in order to ensure excellent stability against overturning of the helicopters which they receive.
It should further be noted that the helicopters placed on board are not, in general, equipped with a landing gear with skids, since these types of undercarriages do not exhibit sufficient elasticity and do not ensure sufficient damping in the vertical direction to permit, under good conditions, a landing on a moving platform, especially since the vertical component of the speed of relative movement of the helicopter and of the platform exceeds 3 m/s. On the other hand, as already set forth hereinabove in relation to the British and American patents, the landing gears with skids do not permit, without the assistance of an auxiliary device, the movement of the helicopters on the alighting platforms with sufficient security, in particular with regard to braking.
For these various reasons, the great majority of helicopters placed on board consists of helicopters equipped with a wheeled landing gear. Having regard to the fairly limited number of helicopters placed on board, the latter are items of equipment which are in general derived from helicopters designed and devised for a utilization from bases situated on solid ground or land-based platforms, possibly on an incline, but never moving. Consequently, these helicopters are therefore equipped with a landing gear of the tricycle type, with a front or rear auxiliary undercarriage, and the two principal and lateral undercarriages of which exhibit a relatively small track, in order to be less heavy and to give rise to an aerodynamic drag which is as small as possible. However, on account of the relatively small track of the principal undercarriages, these tricycle landing gears do not impart to helicopters which are equipped therewith a lateral stability which is sufficient to permit, without a risk of tipping over, especially laterally, the placing of these helicopters on board, on the deck of ships undergoing, with their alighting platforms, intense rolling and pitching movements, as is the case, for example, with frigates and corvettes, with, in addition, the possibility of likewise encountering strong side winds which are troublesome for alightings and takeoffs, as well as for the maneuvers of the helicopters which have been set down, as is the case, for example, on the decks of aircraft carriers (helicopter carriers or airplane carriers).
It is recalled that, when a helicopter wishes to be set down on an inclined land-based platform, it approaches facing the incline and, after having been set down on the platform, it therefore utilizes, in order to maintain itself on the latter, its longitudinal stability which is, in general, far greater than its transverse or lateral stability. However, for a helicopter which has been placed on board and which wishes to alight on a platform which is movable along all axes, this alighting procedure cannot be adopted.
For a better perception of the problem posed, it is recalled that the average lateral static stability of a helicopter having a tricycle gear with a front auxiliary undercarriage is approximately 20.degree.; this substantially signifies that the longitudinal and median plane passing through the center of gravity of the helicopter may be inclined by a maximum angle of 20.degree. to the vertical without risk of lateral tipping-over of the helicopter. Such stability is certainly sufficient for a civil land-based utilization of the helicopter, but the static stability desirable for a helicopter placed on board is approximately 30.degree..
In order to bring the critical angle of lateral inclination to approximately 30.degree. on helicopters having a tricycle gear with a front auxiliary undercarriage, a first evident solution consists in increasing the track in the main undercarriages. However, the improvement of stability by increasing the track of the main undercarriages presents numerous disadvantages, since it necessitates a significant structural modification of the basic helicopter, a certification of the new undercarriages, and it involves an increase of the bulkiness of the helicopter on the ground, a larger aerodynamic drag and thus very substantial losses of performance, as well as the practical obligation to increase the diameter of the useful surface on the alighting platform, and the dimensions of a possible alighting assistance device equipping this platform, for example a device comprising a grid implanted in the central part of the alighting area and cooperating with a harpoon mounted below the helicopter.